Continuous centrifugal casting of tube using liquid mold

ABSTRACT

Continuous centrifugal casting of metal tube on a centrifuged lining of a heavier liquid metal mold, (as lead, tin, or lead-tin alloy). Both the liquid mold material and the molten metal, to be cast to tube, are continuously introduced into the starting end of the centrifuge and continuously exit from the opposite end where the still liquid mold material flows into a suitable catchring for recirculation and the semisolidified or solidified centrifugally cast tube exits axially from the centrifuge for subsequent use as tube or as a basic hollow cylinder for conversion to longitudinal structural items by the technique of collapse deformation.

United States Patent [72] Inventor George R. Leghorn [56] Referen eCited t? f' g'g gg Santa UNITED STATES PATENTS 3 1,831,310 11 1931Lindemuth 164/81 Appl' 768983 2 940 143 6/1960 Daubers m1 164/5 22 FiledOct. 21, 1968 i i Y 45 Patented Nov. 2, 1971 FOREIGN PATENTSContinuation-impart of application Ser. No. 22,708 1 H1896 Great Britain164/81 ir' g k zgg Primary Examiner-R. Spencer Annear 8 ayAtt0rr1eyLawrence Fleming ABSTRACT: Continuous centrifugal casting ofmetal tube on a centrifuged lining of a heavier liquid metal mold, (aslead, [54] gg gi aiifggg CASTmG 0F TUBE tin, or lead-tin alloy). Boththe liquid mold material and the g I Q 3 D molten metal, to be cast totube, are continuously introduced 1C almsl rawmg into the starting endof the centrifuge and continuously exit [52] U.S.Cl 164/84, from theopposite end where the still liquid mold material l6 4/ 6 4 flows into asuitable catch-ring for recirculation and the semis- [51] Int. Cl B22dll/00, olidified or solidified centrifugally cast tube exits axiallyfrom B22d 13/02 the centrifuge for subsequent use as tube or as a basichollow [50] Field of Search 164/82, 84, cylinder for conversion tolongitudinal structural items by the technique of collapse deformation.

solidified casting \\\\\\\Q Q a5 liquid mold PATENTEU 2 SHEET 30F 6PATENTEUNDV 2 Ian 3. 6 1 6 .842 SHEET 86F 6 INVENTOR.

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GzmeczE. LEG/10m B MW% CONTHN UOUS CENTMFUGAL CASTING F TUBE USINGLlQUllD MOLD This application is a continuation-in-part of myapplication Ser. No. 538,506, filed Feb. 11, 1966, now U.S. Pat. No.3,445,922 issued May 27, 1969.

BACKGROUND OF INVENTION A great many techniques for the casting of tubesare known and used in the metal casting industry and most .of thesetechniques have long been in public domain. More than this, it has longbeen obvious that a means of continuously casting such tubing wouldpermit great economy to be realized in the manufacture of suchhollow-ware.

One of the earliest attempts for the casting of tubing on a continuousbasis is exemplified in British Pat. No. 15,912, issued to Lane andChamberlain in 1891. This invention attempted to continuously casttubing by the expedient of continuously pouring the molten metal (to becast) into one end of a solid-wall centrifugal mold and continuouslyremoving the solidified tube from the other end. The technique, whilemeritorious in conception, failed primarily as a result of the highfrictional contact between the mold bore ID. and the cast tube 0D.

A number of other patents teach the continuous centrifugal casting oftube in a solid wall mold by the basic technique of Lane andChamberlain. These include U.S. Pat. Nos. 777,559 to 777,562 issued toStravs and Jager in 1904 (this series of patents disclosed bothhorizontal and vertically downward extraction of the tubes so cast);U.S. Pat. No. 950,884 issued to Winner in 1910 (in this method, asuperimposed slinging action was utilized to continuously force thecentrifugally cast tube from the mold bore); U.S. Pat. No. 1,223,676issued to De Lavaud in 1917 which teaches the use of a rotary mold and aroller disposed within said mold as well as a means for continuouslyejecting the casting as formed; U.S. Pat. No. 2,752,648 issued to Robertin 1956 (this is essentially a repeat of the methods of Stravs and lageras taught in their patent disclosures in 1904 and utilizes canted rollsto extract the centrifugally cast tube downwardly from a vertical mold);and, lastly, British Pat. No. 984,053 issued in 1963 which teaches thedownward extraction of a centrifugally cast tube from a verticalcentrifuge having an internal offset and tapered rotating core mold.

Whereas the foregoing processes have been made to work and producetubing in a continuous manner, they have the drawback of exceptionallyhigh frictional forces between the mold wall LD. and the cast tube OD.as a result of the outward forces on the molten and solidified tubemetal clue to the centrifugal action. Conventionally horizontalcentrifugal casting is done between rotational speeds which produce from50 to 100 gravities of centrifugal force (a 1 pound mass of metal wouldeffectively weigh 50 pounds when centrifuged at the rotational speed of50 G's necessary to produce a dense sound casting and to prevent rainingand sloshing of the molten metal) and, as a result, the extraction ofthe continuously centrifugally cast tube from the bore of the solid wallmold is extremely difficult. With metal wall molds, the exceptional wallfriction causes circumferential splits in the tubing so cast and suchsplits have resulted in the exiting tube being pulled out of the bore ofthe mold as a broken off length instead of continuously. To correct thisdefect, one aspect of the Maxim patent of 1895 (British Pat. No. 22,708)pertained to the use of slippery refractory materials such as axiallyaligned asbestos fibers compacted with plumbago (graphite). Suchslippery and refractory linings greatly increase the workability ofsolid wall centrifugal molds for continuous casting; however, these samecentrifugally created frictional forces cause exceptionally high wearrates on such soft materials. Once an annular circumferential depressionhas been worn into the l.D. of the centrifugal casting mold at thestarting end, a tube is cast having too large a diameter to permitextraction from the exit end. In practice, this is a steady wear processand the bore keeps opening up the solidified tube is extracted from themold. For the casting of steel, the wear rates can be extremely rapidand economically disadvantageous.

Due to the foregoing detrimental aspects of solid wall continuouscentrifugal casting molds a number of nonrotating methods for thecontinuous casting of tube have been conceived. These are invariablybased on the use of concentric inner and outer solid mold walls that arecooled by various means. Such devices are best exemplified by U.S. Pat.No. 2,473,221 issued to Rossi in 1949 (wherein a cast tube is withdrawnvertically downwards from such a mold) and U.S. Pat. 3,022,552 issued toTessman in 1962 (wherein the tube cast between such concentric molds ishorizontally forced out of the casting apparatus by the hydrostaticpressure of the molten metal being cast). Both of these processesutilize nontapered internal (I.D.) molds and as :such, are extremelydifficult to operate on a continuous basis due to the cast tubeshrinking inwardly (thermal contraction) onto the solid mold in thebore. Shrink fits are used to prevent concentrically assembled itemsfrom slipping and, in the case of continuously cast tube, the shrink fitcan cause complete stoppage or rupture of the cast tube. In order toobviate the foregoing problem, the inner concentric mold has beentapered so that the cast tube moves to a smaller diameter portion of the[.D. mold as it contracts of the concentric molds are made as short aspossible. Such tubing is being successfully continuously cast byutilizing either tapered or very short [.D. molds. However, the outputrates (withdrawal rates) are fairly slow and must be carefullycontrolled to prevent either shrinkage binding (too slow a withdrawal)or molten metal seepage (too fast a withdrawal).

The foregoing processes, where used, are of economical value due to theincreased cost of tube and pipe as an end item.

The obvious commercial advantage of being able to continuously casttubing by the centrifugal method (no internal mold required; and withresultant high-integrity pressure cast metal) led to the invention ofthe liquid wall centrifugal mold by Hiram and Hudson Maxim. The processwas disclosed in 1895 in British Pat. No. 22,708. Essentially, the Maximinvention consists of a horizontal centrifugal mold (as in the Lane andChamberlain process of 1891) with the exception that the bore [.D. atthe back part of the mold is greater than at the exit orifice LB. andthe resulting shallow annular depression is filled with a centrifugedlining of liquid lead which extends to the exit end of the rotatingmold. The ID. of the liquid lead lining is substantially equal to theID. of the exit orifice. Molten steel is centrifugally cast onto theliquid lead lining of the apparatus and forms a molten steel cylinderthereon. As the molten steel solidifies to a hollow cylinder (due toheat extraction by and through the liquid lead), as it is forced towardsthe exit end of the mold, it shrinks diametrically due to the ther' malcontraction of the steel and is thus capable of being withdrawn fromwithin the solid exit lip of the rotating mold. The Maxim process(British Pat. No. 22,708) is innovated to greatly increase its productrange and rate of output as, also, is that of Daubersy et al. (U.S. Pat.No. 2,940,143). The Maxim process is a perfectly valid one but 1 havediscovered that it has severe limitations as to the ratio ofwall-thickness to diameter of tubing which it can produce, as explainedhereinafter.

The knowledgeable detail of the Mlaxim patent disclosure attests to theextensive developmental work carried out on the process. As an example,the Maxims provided a hot zone at the starting end by surrounding therotary mold in that area with a furnace. One of the drawbacks to thesuccessful operation of such a device results from the: fact that moltensteel, when poured directly onto liquid lead even when it is heated to afuming temperature, will chill so rapidly that the hardened steelinterface on the liquid lead is rough and knobby and, as such, caneffectively increase the diameter of the cast tube so as to prevent itsremoval from the exit end of the centrifugal tube caster. This fastchilling effect depends on the thermal conductivity of the steel(whether it is less or greater than that Density of solidifying steel ofliquid lead) and, also, on the thickness of the steel layer. For a steelhaving a considerably higher thermal conductivity than lead and wherethe wall (layer of steel) is fairly thick, the solidified steel skinwill remelt and smooth out. This product area (tubes having fairly thickwalls), however, is primarily denied to the Maxim process by the D=65Tlimitation of formula 1, hereinafter set forth.

The patent also discloses pouring on the down-going sidewall of thecaster and the use of vanes to bring the poured in steel into rapidrotation. Such techniques help to obviate the knobby surface caused bytoo rapid chilling of the pouredin steel by sluicing the steel onto theliquid lead.

it is quite probable that this process was ahead of its time as far asavailability of suitable engineering structural materials was concerned.it should be noted that the heated steel (the structural material thenavailable) walls at the starting or hot end of the apparatus would soonfail by creep under the high G(as 70 Gravities) forces and an internalload of layered liquid lead and molten steel.

In a static casting (such as one made in a conventional sand mold), thetotal contraction depends on the solidification contraction and thethermal contraction. In a centrifugal casting (operating at the high G,"gravitational, forces necessary to produce a dense casting) thesolidification shrinkage is nonex istant since, as the denser solidgrains grow from the molten matrix of the surrounding liquid steel, theyare centrifuged to the outer surface and form a solid ring of weldedparticles which have already undergone their solidification contractionprior to uniting into a solidified ring. More than this, the thinsolidified ring is in a highly pliable condition at a temperature justbelow its melting point and is readily stretched to its maximumequilibrium diameter under the centrifugal forces involved.solidification contraction occurs; however, it is evidenced as adecrease in the wall thickness of the solidifying tube while the outsidediameter remains essentially unchanged. From then on the onlycontraction is the thermal contraction of the solidified ring as itstemperature is lowered, by heat abstraction, from the solidificationtemperature of about 1,500 C. to just above the melting point of lead(330 C. which is the minimum allowable cooling for such a system usinglead as the liquid mold.

The distance that the nearly solidified steel (a steel of about 0.20percent carbon is used for illustrative purposes since this is the rangeof greatest commercial output) will sink into the liquid lead willdepend on the density of the steel (7.30 g./cc.) at the l,500 C.solidification temperature, the thickness of the semisolidified steellayer, and the density of the liquid lead that is being displaced(hotter liquid lead will be less dense and the solidifying steel willsink deeper into it). If, therefore, we take the liquid lead at itsgreatest density (10.66 g./cc. at just above its melting point or 330C.) we can determine the minimum amount that the steel will sink intothe liquid lead as follows:

X 100 equals percent of layer thickness of steel that sinks into theliquid lead Density of cool liquid lead In other words, the solidifyingsteel will sink into the cool liquid lead by an amount that is equal, atleast to two-thirds of its own layer thickness while for hotter, lessdense, liquid lead, the solidifying steel will sink in even more.

It is obvious also that, in order for the solid steel tube (which hasbeen centrifugally cast upon the liquid lead mold) to be capable ofextraction from the fixed exit diameter of the mold, the tube radiuswill have to contract by at least the distance which it sank into theliquid lead. The specific volume thermal contraction (the only effectivecontraction in the centrifugal process) between the l,500 C.solidification temperature of the steel down to just above the meltingpoint of lead or 330 C. is 6 percent as shown in FIG. 1. Since the X 100equals 68% or just over 2/3.

linear diametrical contraction is one-third of the volume contraction,the diameter of the tube will shrink by one-third of 6 or 2 percent ingoing from the solidifying tube at l,500 C. to the cool tube exitingfrom the centrifugal casters exit orifice at 330C. A A

Any tube centrifugally cast by the Maxim process which has a solidifyingsteel wall thickness of T units and an exit diameter of D" units will belimited (as to the minimum diameter of tube that can be cast relative tothe solidification wall thickness) in accordance with the terms of thefollowing formula:

which is derived as set forth hereinafter.

Since the solidifying steel sinks at least two-thirds of its wallthickness T into the cool liquid lead, the diameter of the solidifyingsteel tube, at the casting or starting end of the liquid mold, will betwice the radial sinkagc of two-thirds T or 4T/3 units greater than theD' units diameter of the exit orifice (which is the same as the ID. ofthe liquid mold material that overflows the exit orifice weir whendisplaced by the molten steel).

Since the linear or diametrical thermal contraction (in going from thesolidifying state at l,500 C. to the solid state at the minimum exittemperature of 330 C.) is 2 percent, then the 2 percent diametricalshrinkage must reduce the diameter to that of the exit orifice D. forexit thereof.

FORMULA l.

D=65T a tube (so cast) would have to be 5 feet and 5 inches in diameterfor a 1 inch wall thickness. in other words, for the Maxim process towork, the minimum diameter of the tube being centrifugally cast must be65 times greater than the wall thickness of the tube as it forms in thecentrifugal casting machine (for a 0.20 percent carbon steel).

There is an important limitation to the lower limit of the temperatureof the exiting steel tube and this is due to the phase transformation ofthe austenite (high-temperature phase) to thermal decomposition productssuch as ferrite and pearlite which starts at about 700 C. In low carbon,low alloy steels this phase transformation takes place in less than 2seconds at about 600 C. and is accompanied by a volume growth whichcounteracts and reverses the shrinkage to the extent that a 3-footdiameter tube will quickly experience a diametrical increase of slightlyover l/l6th inch. Such an expansion can cause jamming of the taperedtube into the outlet orifice of the mold with catastrophic results.

The thermal specific volume contraction graph for a low carbon steel isshown in FIG. 1 and clearly illustrates the volume expansion between 700and 500 C. on cooling.

Due to the danger ofjamming as a result of this sudden expansion (due tophase transformation) of the exiting tube, the minimum exit temperatureshould be at 700 C. or above for such steels.

With reference to FIG. 1, the scaled volume contraction distances froml,500 C. to room temperature to that in going from l,500 to 700 C. is0.009l/0.0058 and the 0.009] distance is equal to the 7.2 percent volumecontraction. Therefore the percent volume contraction (V) is going from1,500 to 700 C. is 0.009l/0.0058=%/v% or V=0.0058/0 .0091 (7.2%)=4.58%

The linear or diametrical contraction is one-third of the volumecontraction of4.58/3=l .5 3%.

The release of the heat of solidification of steel would raise thetemperature of the cool lead to at least (actually to a much highertemperature) 530 C. and the density of liquid lead at 530 C. is 10.420g./cc.

The solidifying steel (density 7.30 at l,500 C.) will sink into the lead(density of 10,420 at 530 C.) to 7.30/l0.420X l00=70% or 0.07 T (where Tis the layer thickness of the solidifying steel tube.

Therefore, by a rederivation of formula 1, we have the following:expression reads: (D+2X0.7OT) l(D+2 0.7OT)-l.53(D+2X0.70T)=1OOD 100D+l40Tl.53D 2.l4T=l00D 1.5 3D=l40T-2.l4T 1.53%13186T FORMULA 2 It canreadily be realized by this ratio of tube wall thickness to diameter (tojust let the tube clear the exit orifice of the mold at 700 C.) that theMaxim process is subject to some very severe limitations of productoutput. It may well be that this reexpansion of steel, which began at700 C. on cooling, was unknown in 1895 and it may have been too severe ahurdle for the Maxim process in its developmental state.

FIG. 2 illustrates the limitation on product output due to the diameterof the low-carbon steel tube, produced by theMaxim process, having to be65 or 90 times as great as the wall thickness of the cast metal as itsolidifies.

An even more restrictive limitation on the rate of output derives fromthe fact that the wall of the tube must bealmost completely solidifiedbefore any product at all can be extracted from the centrifuge exitorifice since, under the high G forces of a centrifugal caster, thesolidification contraction (in going from liquid to solid at thesolidification temperature) is practically nil. In order to increase theoutput rate of such a continuous tube caster, it would be highlydesirable to have the tube exit from the apparatus with a solid outershell and an inner shell of still molten steel. In this manner, theouter shell could be immediately chilled with multiple sprays or jets ofa cooling liquid (as cool liquid lead or mixed hydrocarbons and water asin the Maxim system whereby the inventors attempted to correct thislimitation to some extent by internal spray cooling) for more rapidtheatextraction andgreatly increased product output.

Another process in which a steel tube is cast on a centrifuged mold ofliquid lead is the subject of US. Pat. No. 1,831,310 issued to Lindemuthin 1931. Basically, it is disclosed in one portion of the Maxim patentbut includes some slight improvements thereon such as glass additions tothe l.D. surface of the centrifugally cast tube.

The latest patent for the continuous centrifugal casting of tube incontact with a liquid lead lining is U.S. Pat. No. 2,940,143 issued toDaubersy and Schlemmer in 1960. Basically, this Daubersy et al., systemuses continual small additions of lead to the system so that the solidcentrifugally cast tube is permitted to exit from the bore of the casteron'a thin lubricating film oflead.

Analysis of the Maxim Process as Disclosed in British Pat. No. 22,708:

The Maxim patent (line 5 of page 2) states The dam at the forward orexit end of the cylinder has a height or depth approximating thethickness of the fluid bed in order to prevent the latter from flowingout at that end and to prevent an excess of waste of the fluid bed overthe surface of the dam. The fluid bed may be replenished or maintainedas fast as it becomes deteriorated or waster, by feeding into therearward end of the cylinder an additional quantity of lead or othersubstance of which the bed is composed. The lead thereby supplying thefluid bed with fresh material.

From the Maxim patent starting at line of page 2The iron or other pipeformed upon the fluid bed is preferably solidified by cooling at a pointconsiderably to the rear of the dam at the exit end of the cylinder, sothat it shall contract sufficiently upon cooling to give it clearance inorder that it may pass freely over the dam at the forward or exit end ofthe cylinder."

With respect to the foregoing prior art, it is evident that dragout ofliquid lead occurred in the system (and this would have attendantlubricating characteristics) and that such Lil losses, along with thosefrom other sources, were continually made up by lead additions. TheMaxims attempted to avoid such dragout by solidifying the cast tubeconsiderably to the rear of the exit dam whereas, in the Daubersypatent, the escape (leakage flow) ofa small amount of lead is encouragedby the continuous additions of small amounts of lead to the system so asto form a lubricating film of liquid lead between the darn lip and theoutside surface of the exiting tube.

The Maxim patent includes means for controlling the rate of extractionof the tube (this permits solidification and subsequent thermalcontraction considerably to the rear of the dam so as to avoid anypossibility of the solidified tube jamming against the exit orifice damby too fast or uncontrolled withdrawal) as in lines 50 to 53 of page 2as follows: We also provide means for drawing the pipe from the rotatingcylinder as fast as it is formed, that is, as fast as it becomessolidified, these means preferably consist of friction rollers set at asuitable angle. Given a constant speed of rotation, a constant speed ofcharging with molten metal, and a constant means for cooling, thethickness of the pipe or tube formed will be governed by the rapiditywith which it is drawn from the forming cylinder."

The process of the Maxim patent is operable within the restrictions ofthe D=65Twall thickness to tube diameter ratio for steel not exhibitinga phase change and D=9OTfor low carbon lowalloy steels which undergo arapid phase change expansion. The method is entirely workable when useis made of the more advanced structural material available today.

It should be noted, however, that inadvertent changes in liquid levels,or too rapid chilling of the inpouring molten casting metal, or toorapid (or too slow, if the resulting increased wall thickness exceed theD to T ratio) a withdrawal of the tube will result in jamming at theexit orifice which will result in stoppage at the very least.

Analysis of the Daubersy Process as Disclosed in U.S. Pat. No.2,940,143:

The Daubersy patent is essentially a variation of the Maxim process bythe method of liquid lead additions and by the interior design of thesolid portions of the centrifuge mold wall in an attempt to create aself-regulating effect over inadver' tent diamctrical changes of thecentrifugally cast molten metal.

Essentially, the Daubersy patent combines a shortened dry wall mold(this is a technique for cutting down on the excessive frictional forcesthat attend a complete centrifugal dry wall mold) with an extendedliquid wall mold of the Maxim type. A hot, thin, malleable shell ofsolidified steel forms on the interior surface of the dry wall mold andis then forced off of the drywall mold by the head of molten steel andonto a liquid mold which then acts as a heat extracting medium forsolidifying the balance of the molten steel of the centrifugally formedtube. From this point on, the process is the same as the Maxim processexcept that the lubricating film of lead is maintained by continuousadditions of small amounts of cool liquid lead at the starting end. Thehead of inpouring molten steel forces the solidified steel tube out ofthe bore of the casting machine and is facilitated in doing so by thelubricating film of lead which lines the bore of the caster. The castingmachine has no positiveimeans of extracting the solidified tube as inthe Maxim process but depends on the pushing action of the head ofmolten steel at the starting end. Further, and as clearly stated inthepatent, the exit orifice of the mold conforms closely to the outsidediameter of the cast tube after the piece has accomplished itsshrinkage. (Lines 22-24 of column 2 of the Daubersy patent, and whichrefer to the dam or exit orifice lip of the patent drawings, notes the ris the shrinkage when the centrifuged annular piece has accomplished itsshrinkage.

in all four patent drawings, the dam of the centrifugal tube caster isshown as being inwards from the CD. of the just solidifying tube (at thestarting end) by the amount of shrinkage r and except for a thinlubricating flow of liquid lead, the dam l.D. is in close contact withthe CD. of the tube (Annular piece" as it is called in the patent) whichhas accomplished its shrinkage.

It can be seen from the foregoing that the dam at the exit orificeextends to within lubricating contact of the tube after shrinkage hasbeen accomplished and is, therefore, the same as in the Maxim process.The Maxim and the Daubersy processed depend on thermal shrinkage of thetube to permit its egress from the system. In this manner, the primarymode of escape of the cast tube is by thermal shrinkage and in thisrespect, the process is subject to the requirement of completesolidification of the tube wall prior to its exit from the system.

RESUME OF THE INVENTION The present invention. provides for introductionof moderate (approximately to 25 percent by weight of the metal beingcast to tube) to large (over 25 percent by weight of the tube metal)amounts of liquid mold material and the maintenance of the outsidediameter of the molten metal cylinder equal to, or less than, the exitorifice diameter of the centrifuge, which permits the cast tube to floatout of the bore of the centrifuge on an axially flowing stream of liquidmold material. Due to the freedom inherent to the floating action, suchexceptional rates of output are permissible that the process issuperior, on a tonnage per hour basis, to the currently used continuouscasting processes.

THE CONTINUOUS CENTRIFUGAL CASTING PROCESS OF THE INVENTION IN GENERALThe process of my invention is designed to greatly extend the limitedrange of tube product output inherent in the Maxim process. It hasbecome apparent to me that the ratio of wall thickness to diameter oftubing that can be produced by this process is severely limited due tothe fact that solidifying steel at its solidification temperature (about1,500 C. for a 0.2 percent carbon steel) will sink into the liquid leaduntil it has displaced its own weight of the liquid mold material (theArchimedes principle).

This limitation of product output range (for a low-carbon, low-alloysteel) which is expressed by the general formulas D=65T and D=9OT, andis shown in FIG. 2, is not applicable to my process. As an example, theMaxim process cannot produce a mild steel tube having a one inch thickwall and a diameter of less than five feet. On the other hand, myprocess can produce such tubes of one inch wall thicknesses havingdiameters of less than one foot. This ability to cast fairly heavywalled tube in small diameters is particularly important where the tubeis to be used as a basic starting point for the manufacture oflongitudinal structural items (by collapse deformation thereof) such asI-beams channels, angles, etc.

This advantage of my process derives from the technique of restrictingthe outside diameter of the molten tube so that it does not exceed thediameter of the exit orifice of the centrifugal continuous castingmachine. In this manner, the cast tube can float out of the bore of thecasting machine on an axially flowing cylindrical stream of liquid moldmaterial. In fact, the exiting speed of the flowing liquid mold material(under the action of the high G, psuedogravitational, leveling force ofthe centrifuge) can be so rapid that the casting machine must beexceptionally long in order to permit sufficient time for solidificationof the molten metal; or, the flow of the liquid mold material and thespeed of exit of the solidified tube must be restricted to allowsufficient time for heat extraction and solidification. In actualpractice, the exiting speed of the centrifugally cast tube is controlledby conventional means (such as a modification of that used in the Maximprocess) and the flow of liquid mold material is restricted (byappropriate downstream location of the annular overflow dam or weir) sothat advantageous casting rates are obtained with a moderate lengthcasting machine. At the same time, the casting machine is segmentized sothat it can be readily extended to greater lengths so as to greatlyincrease the rate of output as demanded. More than this, the castingmachine is unitized so that a centrifugal casting cylinder, permittingoutputs of larger or smaller diameter tubes, can be readily exchangedand still utilize the same basic mechanisms or rotation, extraction.cutoff, molten metal and liquid mold introduction and exit and the like.Segmented molds and unitized construction have long been known inbatch-type centrifugal casting.

The restriction of the outside diameter of the molten metal tube so thatit does not exceed the exit orifice diameter of the centrifugal castingmachine (whether an annular weir is present or not) has a very importantadvantage since it obviates the danger of a solidified tube (having adiameter that is greater than the exit orifice, as in the Maxim andDaubersy processes) jamming the exit port and creating an expensivestoppage or a catastrophic failure. These casting machines, of Maxim andDaubersy, can effectively and continuously produce centrifugally casttube (when the outside diameter of the molten tube is greater than thediameter of the exit orifice) providing that the exiting speed of thesolidified tube is carefully controlled. However, a slippage or otheruncontrolled extraction failure can result in an expensive stoppage orhazardous condition and it is for this reason (as well as to increasethe scope and speed of product output) that the diametrical restrictionsof the disclosed process are imposed.

It should be noted that the segmentized and unitized construction of themachine permits a rapid and easy change of product output (as from alarge diameter thin-walled tube to a small diameter moderate-walledtube) with the same basic mechanism and it is a primary intention ofthis process to produce such tube variations as basic items for the moreeconomical production of other longitudinal structural shapes (as plateby the collapse of large diameter thin-walled tube or railroad rails byinwardly collapsing moderate-wall small diameter tubes and roll-weldingthe contiguous interior surfaces while sizing the collapsed structure tothe final desired item of longitudinal structure).

It should be noted further that my process permits the continuouscentrifugal casting of tubes having smaller diameters than is nowfeasible by batch type solid wall centrifugal casting. In batch typecentrifugal casting there are certain limitations as to the length oftube that can be cast for particular diameter and this is particularlytrue for tubes having small diameters (as less than two inches OD). Thelongitudinal contraction of such tubes, in cooling down from thejust-cast to the extraction temperature is sufficiently great that thecircumferential rupture will occur if this shrinkage is undulyrestrained. Such restraint is produced by minor ovalness, or out-of-lineof the bore of the centrifuge, end sticking, or surface roughness. Insmall diameter tubes, the diametrical shrinkage is insufficient toobviate (shrink away from) such restraining mechanisms and the largeamount of rejections due to such circumferential rupture makes suchproduction uneconomical. My process can produce such small diametertubes on a continuous basis and without ruptures since the onlyrestraint to longitudinal shrinkage would be the shearing forces in theliquid mold material and these are reasonably small. Such small tubescan contract both longitudinally and diametrically without damaginghinderance.

As stated previously, one of the primary features that differentiates myprocess from those of Maxim and Daubersy is the fact that the CD. of thejust solidifying contrifugally cast tube is maintained equal-to orless-than the ID. of the exit oriflce of the centrifugal caster. Inorder to accomplish this criterion, certain physical requirements mustbe met. As a specific example of the operation of the present invention,in that the disclosed process can continuously centrifugally cast mildsteel tubes having a wall thickness of one inch and an OD. of less thanone foot, the following example is presented:

The mild steel tube being cast has an CD. of 10 inches and an ID. of 8inches (wall thickness of 1 inch) and is centrifugally cast at arotational speed which is equivalent to 50 Gs (gravities). At thesolidification temperature of l,500 C., the density of thejust-solidifying steel is 7.30 g./cc. or 0.264 lbs/cu. in. However,since the casting is being carried out at 50 G's. the effective weightof a cubic inch of the steel (at l,500 C.) is 50X0.264 or 13.2 pounds.But, if we project a square inch area of surface on the CD. of the tubeonto the tube axis, we have a truncated wedge removed from the tube wallwhich has an exterior surface area of 1 sq. in. and an interior surfacearea of 0.8 sq. inches, along with a radial (wall thickness) depth of 1inch. The volume of this truncated wedge is 0.9 cubic inches and thisvolume of semimolten metal bears on the onesquare inch of outer surfacedue to the centrifugal action. Since one cubic inch of the metal weights13.2

pounds, then the 0.9 cubic inches of the truncated wedge will have aneffective weight of 0.9X1 3.2 or 11.9 pounds at 50 Gs and this weight isexerted against the one square inch of area on the OD. surface andcreates a pressure of l 1.9 p.s.i.

Therefore, in order to prevent the 1 inch semimolten steel layer fromsinking into the liquid mold material (as liquid lead), aback pressureon the liquid lead (in excess of its normal pressure at 50 GS) of 11.9p.s.i. must be accomplished. Such a back pressure of 11.9 p.s.i. willcounter-balance the weight of the 1 inch steel layer and reduce theoutside diameter of the semisolid steel tube to that of the annularoverflow weir (exit orifice l.D.).

SPECIFIC EXAMPLES OF METHOD This necessary back pressure can, and is,created by any one or any combination or permutation of the followingfive species of my method (which will herein be designated as Method 1,Method 2, Method 5.

Method 1. By restricting the liquid mold exit orifice (the annular gapbetween the solidified tube OD. and the the centrifuges exit orificel.D.

Method 2. By extending the length of the weir (exit orifice) lip to asufficiently great extent that the required down-stream line pressuredrop of l 1.9 p.s.i. is experienced.

Method 3. By creating a vacuum within the steel tube thatcounterbalances the weight of the 1 inch thick layer of steel at 50 Gs.

Method 4. By raising the atmospheric pressure (exterior'to the tube andthe exit orifice or at the entrance end and exterior to the vacuum sealmeans) by the desired amount over that of the ambient atmosphericpressure (14.7 p.s.i. is the average sea level atmospheric pressure and14.7+l 1.9 or 26.6 p.s.i.a. would be required under normal conditions).This is actually done by surrounding the exiting steel tube and theexit-end of the centrifuge with a suitable enclosure and introducing aninert dry gas therein under the desired 26.6 p.s.i. of pressure.

Method 5. By reducing the rotational speed of the centrifuge so as todecrease the centrifugal force (number ofGs). Actually, 50Gs, while notthe lower limit of rotational speed necessary to prevent raining andsloshing of the liquid contents of the centrifuge, normally consideredthe lower limit for the production of a dense defect-free casting.

With respect to Methods 1 and 2, it can be noted that these methods areentirely feasible. However, a large amount of liquid mold material mustbe introduced into the system to maintain the desired back pressure atan equilibrium value. As a approximation (depending on restriction ofthe exit orifice and the length or line drop of the weir lip) thethrough-put weight of the liquid mold material must be equivalent to thecasting output. Since this can be greatly in excess of 100 tons of steelper hour, it can be realized that a large amount of liquid mold materialis required. Even where Methods 1 and 2 are used in combination and aminimum rotational speed of 50 Us (for the desired density of casting)is used, moderate amounts (such as percent by weight of the cast metalthrough-put) of liquid mold material must be continuously introducedinto the system. For this purpose, the Methods of l and 2 are not thepreferred means of producing the desired back pressure even though theyare present to some extent in any system where the liquid mold materialexits continuously from the system.

Method 3, the creation of a partial vacuum on the interior of the tube,is the preferred method since it is the most forceful means ofaccomplishing the desired back pressure and introduces other beneficialeffects as well. In a prior US. patent application, Ser. No. 538,506,the use of an internal vacuum within the centrifugally cast tube hasbeen described in con junction with the continuous collapse forming thetube to longitudinal items of structure. In such a process, the tubecavity is sealed at the exiting end by the inward collapse of the tubewalls and the welding together of the inner contiguous surfaces of thetube wall. The seal at the starting (pouring) end of .the centrifuge iscreated by a nonrotating end plate, or disc,

then applying the suction, the gases given off by the molten metal areof a-reducing or inert nature (as carbon-monoxide, hydrogen andnitrogen) and these gases maintain the inner surfaces of the tube inabright oxide-free condition which permits and facilitates thepressure-welding of the contiguous interior surfaces of the tube one tothe: other. It may be mentioned thatthese same gases create porosity orblowholes in ingots cast-the the old ingot-mold process and that thesegas cavities are collapsed to a defect-free solid condition bysubsequent rolling which welds the clean oxide-free inner surfaces ofthe pockets together. This is but one ofthe majoradvantages from the useof a partial vacuum internal to the tube being cast.

b. Where the internal vacuum is sufficiently great that a positive forcemust be exerted by the axially aligned pullout mechanism the tension onthe tube aids in preventing axial warpage thereof.

C. The liquid mold material has less chance of oxidation since no airisinternal to the casting chamber.

d. The internal partial vacuum materially aids the collapse formingoperation.

e. The internal partial pressure of reducing gases can be maintainedinterior to the tube, as will be revealed in the teachings of thisinventions disclosure, for as long as desired and the internal surfacesof the tube will remain bright and oxide free for subsequent reheatingof the tube and collapse deformation thereof or, if desired, as aprecleaned surface for subsequent application of an internal oxidationresistant coating of enamel, plastic, rubber, zinc, tin, aluminum, leador the like.

In the Method 4, the volume external to the exit end of the centrifugeis enclosed to afford an effective seal which permits the applicationofa higher than ambient gas pressure which forces the liquid mold materialto back up in the centrifugal caster until the ID. of the liquid mold isequal-to or less-than the ID. of the centrifuges orifice. Thispressurization is accomplished with adry, inert gas such as nitrogen,argon, helium, or the like. it is preferred to use this method inconjunction with the Method of number 3, since, by this combination, thewall thickness of the 10 inch (0 .D.) tube can be considerably in excessof one inch.

As an example: A partial vacuum of 12 p.s.i. (2.7 p.s.i. absolute) lessthan the ambient air pressure 14.7 p.s.i.) internal to the tube and anexcess pressure of 6 p.s.i. (20.7 p.s.i. gage) external to the tubewould create an additive effective pressure (for decreasing the OD. ofthe molten metal tube) of 18 p.s.i. Since, at the 50 Gs used forcentrifuging, 1 cubic inch of the solidifying metal (mild steel) wouldweigh 13.2 lbs./in and, therefore, the 18 p.s.i. would support athickness of l8/l3.2 or 1.36 inches of the steel. Actually, due to thetruncated wedge section, the layer thickness of molten steel sup portedby the additive l8 p.s.i. would be calculated as follows:

If the layer of molten steel was flat, the 18 p.s.i. would support alayer thickness of 1.36 inches. A truncated wedge section (FIG. 3) ofsuch a 10 inch diameter tube would have considerably less volume (forthe same layer depth) bearing on the square inch of the tubes OD. and,for the same volume of 1.36 cubic inches, would have a greater radialdepth of molten metal. If we let this layer thickness of the tube bedesignated by T, then the radius of the ID. of the tube is inches T.

If we let L designate the length of the ID. are intersected by theprojection of the one inch long are (the 1 inch long circumferentialside of the square inch of area on the tubes O.D.), then l/L =S/S-T andL=5T/5 But, the volume of the truncated wedge is equal to L+1l2 T l andthis is equal to the 1.36 cubic inches of circumferentially layeredsteel. Therefore, L l

X T= 1.36 or (L+1)T=2.72

In other words, an 18 p.s.i. excess pressure due to a combination of aninternal vacuum and an external positive pressure will counterbalance a1.62-inch layer thickness of mild steel tube having a inch O.D.

This same excess of pressure (18 p.s.i.) can also be used tocounterbalance a less thick layer at a higher G rotational speed (as1.08 inch at 75 Gs).

Aluminum, with a density of about one-third that of steel, can beproduced as a 10 inch O.D. tube having a wall thickness some three timesthat of steel under the same, forementioned, conditions of centrifuging.In the case of such metals as aluminum and copper (which, unlike steelsand irons, have a considerable solubility in the liquid mold materialsin the molten state), it is preferred to minimize the layer thicknessand employ higher rotational speeds (G forces) since such high G forcespromote the layering effect and greatly ameliorate the tendency forintermixing and allowing between the molten metal being cast and theliquid mold material. This technique, of minimizing the wall thicknessand utilizing high G forces to accentuate the layering effect, permitsthe continuous centrifugal tube casting of such metals as titanium andzirconium onto a liquid mold of tin. Due to the reactiveness of suchmetals with refractory conduit materials, however, solid rods of thesemetals are arc-melted in the interior of the inert-gas-purged andevacuated tube cavity to produce the desired molten metal.

Method 4 has the further advantage of preventing any oxidation of theliquid mold material (as lead, lead-tin) since the liquid mold materialis protected by the inert gas of the external enclosure. Also, thehigher than ambient pressure of the inert gas helps to suppress thevaporizing tendency of the liquid mold at the exit or overflow-end ofthe centrifuge.

Method 5 has already been discussed. Less than 50 6'5 can be used but itis not particularly desirable unless a very heavy wall thickness ismandatory.

It should be noted that batch-type centrifugal casting is an old andwell-established art. Such parameters as the rotational speed necessaryto produce a specific G force for a specific mold diameter are wellknown as, also, are the lower and upper practical limits of G forces(rotational speeds) utilized. it is sufficient to note herein that thesupporting action of the liquid mold material on the outer surface ofthe tube being centrifugally cast (and, also, the use of Method 3 and/orMethod 4) permits the use of much higher rotational speeds (G forces)than is permissible with a conventional dry-wall centrifugal mold.

With respect to the methods 3 and 4, it is preferred to utilize higherinternal vacuums (Method 3) and lower external positive pressure (method4) where tubes having a smaller diameter and heavier wall thickness isconcerned. Conversely, in the production of large diameter tubes ofthinner wall section, it is preferred to utilize a much lower internalvacuum (Method 3) and higher external positive pressures (Method 4 incombination. The reason for this preference is that the ambient pressureof the air (as standard 14.7 p.s.i.) creates a back pressure on the tubewhich is directly proportional to the crosssectional area of the tubeand, also, to the pressure differential between the ambient atmosphericpressure and the internal vacuum. As an example, a tube having a 10 inchO.D. (crosssectional area of 78.5 sq. inches) and an internal vacuum of4.7 p.s.i. (pressure differential of 14.74.7=l0 p.s.i. with regards to astandard atmospheric pressure) would experience a backward thrust of78.5 inFXlO p.s.i. or 785 pounds. in other words, it would require aforce of 785 pounds on the tube to counteract the internal suction andpull the tube out of the bore of the centrifugal casting machine. On theother hand, a large diameter thin-walled tube (30 inches in outsidediameter as an example) would have a cross-sectional area of 709 sq.inches and, if the pressure differential (between the interior vacuumand the ambient pressure) was 10 p.s.i., a force of 7.090 pounds wouldbe required to get the tube out of the bore of the casting machine. Ifthe 30 inch diameter tube had a inch wall thickness and wascentrifugally cast at 50 G's, the pressure differential necessary tocounterbalance the steel would be one-fourth of 13.2 p.s.i. or 3.3p.s.i. In this case, the required 3.3 p.s.i. could be made up entirelyby application of a positive external pressure (method 4) of l4.7+3.3 orl8 p.s.i. and the internal pressure of the 30 inch diameter tube wouldbe 14.7 p.s.i. or the same as the ambient pressure. By this technique, avery small force (supplied by the liquid mold flow) would be required toextract the tube from the bore of the casting machine since the externalpressure (of Method 4) acts on the periphery of the tube to justcounterbalance the weight of the steel tube at 50 G's and does not acton the end (cross-sectional area) of the tube to create a backforcewhich must be overcome (as in Method 3) to get the tube out of thecasters bore.

lt is readily apparent from the foregoing examples that a very widerange of latitude is available to the operator, in the application of aninternal vacuum (Method 3) and an external positive pressure (Method 4),for ready extraction of a tube from the centrifugal casting machine. Ajudicious (readily calculated) selection of internal and externalpressures is available for all practical casting requirements.

All of the foregoing examples have been predicted on the use of aninternal vacuum (Method 3), or an external positive pressure (Method 4),or a combination thereof just counterbalancing the centrifugal weight ofthe layer of metal being cast and, under these circumstances, any slightthermal contraction as in cooling from the l,500 C. solidificationtemperature of mild steel down to a collapse deforming and rollweldingtemperature of about l,l 15 C.) or slight back pressure (as is normallyattendant to such a system by Method l and/or Method 2) is sufficientfor free exit of the tube from the exit orifice.

Actually, by increasing the through-put of liquid mold material for anyfixed conditions of Methods 1 and/or 2, such back pressure quicklyasserts itself and the molten part of the metal tube being cast issqueezed in to a decreased equilibrium O.D.v Due to this combinedaction, of Method 1 and/or 2 in combination with Method 3 and/or 4, theaction of Methods 3 and/or 4 can be considerably less than thatnecessary to make the CD. of the molten tube equal to the ID. of theexit orifice of the centrifugal casting machine. The action of Methods 1and/or 2 can be utilized to further decrease the CD. of the cast tube tothe amount desired for purposes of exit from the system.

It is preferred to utilize Method 3 and/or 4 only to the extent that theOD. of the tube is somewhat greater than the ID. of the exit orifice ofthe casting machine since, by so doing, the tube stays in heat transfercontact with the liquid mold material for a longer period of timeinstead of moving out of contact with the liquid mold due to thermalcontraction. The Methods of 1 and/or 2 are utilized to the small extentnecessary to back up the liquid mold and thus decrease the tube OD. and,at the less steels as an example) having a lower thermal conductivitythan the liquid mold material (as lead) can exhibit a considerablemolten interior lining on exit from the centrifugal casting systemwhereas those tube metals (aluminum, copper, or lowealloy low-carbonsteels as examples) having a thermal conductivity that is greater thanthat of the liquid mold material must be at least in a semisolid stateon the tubes interior (and solid on the OD.) for successful processing.

In the case where the pressure differential of Method 3 and/or 4 issufficiently great to more than just counterbalance the centrifugalweight of the metal being cast, it might be expected that the tube woulddecrease in diameter (which it does) to the extent that it would liftaway from the liquidmold and permit ingress of air or inert gas into thevacuum of the tube's interior via bubbling through the molten zone ofthe tube. This can and does happen, but not immediately beyond the pointwhere the pressure differential overbalances the zero point.

A stable-state condition exists for pressure differentials in excess ofthe zero point and this is due to the wetting action (attraction) of theliquid mold material (especially where tin is present) and the surfacetension of the molten metal being '50 cast. This operating area(pressure differential beyond the zero point) is not actually used sincethe stable-state condition is not that broad and can readily bedestroyed by any out-ofbalance or other vibration producing condition ofthe rotating system. It does, however, afford a usable margin of safetyfor the condition of exact counterbalance.

It is one of the important features of this invention to utilize theadvantageous system of a vacuum internal to the tube being cast (Method3) in the instance wherein tube itself is the end'item instead of alongitudinal structure formed by inwardly collapsing the tube walls overits entire output length. In the practice of making tube for its ownuse, a tube (having a capped or crimped vacuum sealed exit end) is usedas the starting tube so that the desired vacuum (depending on the wallthickness of the tube, the densities of the molten tube metal and theliquid mold, the G force of the centrifuge, and

-the ambient pressure of the atmosphere) can be drawn on the tubeinterior. The machine then continuously produces a long length ofsolidified rotating tube which exits into an axially aligned cradlewhich permits such combined egress and rotation. Such a cradle canrotate with the tube by virtue of the same drive mechanism as that whichrotates the centrifugal casting machine. A multiplicity of axiallyaligned rollers supports the periphery of the tube and, at the sametime, can either permit or cause the tube to move axially away from thecasting machine. In the case where axial movement is permitted, therollers are mere idlers which are attached to and rotate with thecradle. In the case where they cause the tube to move axially, therollers are spring or piston loaded onto the outer surface of the tubeto give a friction drive contact which pulls the tube from the bore ofthe centrifuge as is necessary where an internal vacuum (Method 3),which causes a suc tion, must be opposed. The rollers, in this instance,are suitable driven by sun gears (via a suitable gear cluster system forsuch power transmission) and are activated or deactivated by a suitableclutch mechanism. Such mechanisms are well known to those practiced inthe art of rotary coupling and uncoupling. At the same time, there is anaxial gap in this cradle system, near the exit end of the centrifuge,with appropriate torch reheating means and rotating opposed swaging orforging hammers which move in axial synchronization with the exitingtube and swage or pinches a reheated section of the tube to a vacuumtight closure after any desired length has been produced. The pinch orswage closing mechanism then returns to the initial starting place whereits operation is recommenced after another appropriate length ofvacuum-sealed tube has been produced. Along with the swaging mechanism,and axially further away from the centrifugal caster by any appropriatelength (a 2 foot long swaged section and a 200 foot length of tubebetween swages would limit the loss of tube due to swaging to onepercent), is located an appropriate cutoff device which travels in axialsynchronism with the exiting tube and cut off the tube at the middle ofthe swaged or forged down closure so as not to destroy the integrity ofthe internal vacuum. After cutting the tube in the axial center of theswaged section, the cutoff returns to its starting point for recouplingto the axial travel mechanism and cutoff of the tube section at theappropriate time. By this synchronized and discretely repeatablesequence of swaging-down and subsequently cutting offthe exiting tube,the integrity of the internal vacuum (with its manifold advantages) ismaintained dur ing and after the tube casting operation.

It is convenient to forge-flatten the exiting tube (just as a soda strawcan be pinch-flattened in a selected area between thumb and forefinger)at the separating point. However, even though this serves as a simplemeans of sealing and maintaining the integrity of the internal vacuum,it is the preferred method of this invention to swage or peripherallyhammer forge such separation points to a solid round having its forgewelded centerline coincident with the axis of the tube. These endclosures (after separation of the tube lengths at the midlength of thesolid swaged-down closure) can be cut from the tube ends with anintegral portion of the tube length as long as desired. Such cutoffclosure lengths are conveniently used to fabricated pressure bottles ortanks for oxyacetylene, propane storage and the like. In this manner,the closure part of the tube is not subject to remelt but affords greateconomies in the manufacture of pressure tanks and storage vehicles.

My preferred means for extracting (pulling the tube out of the bore ofthe caster in opposition to the suction of the internal vacuum) is topower the rotating swaging apparatus so that, once it has swaged downthe tube to a vacuumtight solid round, the swaging apparatus remainsgripped to the solid reduced tube closure and pulls the tube out of thebore. The axial travel of the apparatus can be powered by any convenientmeans (such as a chain drive, cogwheel, worm screw, etc.) and can begeared to or be separate from the rotational means as desired. Thesystem utilizes two such swaging down and pullout mechanisms so that,while one mechanism is pulling out the tube, the second mechanism can beswaging down a tube closure some 200 feet closer to the centrifugalcaster. Once the 2nd mechanism has swaged-down and gripped the tubeclosure for powered pullout, the first mechanism (axially further awayfrom the centrifugal caster) the seversthe tube lengths from each otherat the midlength of the swaged-down closure so as not to destroy thevacuum seal. The first mechanism is then returned to the starting pointto restart as the second mechanism. The two mechanisms thus continuallyreplace each other at the starting point. Alternately, by way ofdecreasing the axial floor-space requirements, the swage-down andpullout mechanism can grip the swaged' down end of the tube being pulledout and, at the same time, sever the completed length which then isreleased from the accordion pleat cradle (d series of idler supportswhich pull out at regular intervals to support and align the rotatingtube sections between the swage-down mechanisms) and rolled off at rightangles for storage of processing. This is not the preferred means sincea grip slippage would result in the tube being sucked back into the boreof the caster with attendant destruction of the internal vacuum,increase in the molten metal tube OD. and stoppage of output forrepairs. In the preferred means (using some length, as the 200 footexample, more floor space, depending on the tube lengths produced), anyslippage of the grip merely brings the pullout mechanism into contactwith the belled-down part of the tube and creates a positive and safepullout.

In the foregoing manner, long sections of tube (like straight sausagelinks) are produced which have an internal vacuum of partial nature. Theinternal surfaces of these tube lengths are clean and bright (due to theinert or reducing nature of the gases contained therein) and thispermits the collapse deformation thereof to longitudinal structure (atan appropriate reheat temperature) with roll-welding of the cleancontiguous interior surfaces. The partial interior vacuum, along withthe clean bright interior surfaces, are very effective in promoting theapplication of interior coatings to the tube since (by clipping the tubeend while immersed in the fluid coating media and replugging the openingonce the exact amount of coating has been sucked into the interior ofthe tube'length) the tube can then be rotated-in-place to evenly coatthe tube's interior surface while the coating is being heat-cured,catalytically cured or solidified in place as suits its nature (whetherorganic, nonorganic or metallic). The clean interior surfaces acceptsuch coatings with excellent adhesion.

In the collapse-deformation and roll-welding of such tube, the tubesection can be collapse-formed partially (over its entire length) orcompletely collapse-deformed (over a part of its length), withappropriate preheating, so that a positive internal pressure (aboveambient) is built up inside the tube. The back end of the tube is thenperforated to permit escape of the internal gases for continued hotcollapse-deformation and sizing to a completed item of longitudinalstructure. In this manner, the internal vacuum does not suck in moistair which could contaminate the bright-clean interior surfaces to thedetriment of their being roll-welded together.

The long lengths of tube (they can readily be made as milelong lengthsby exiting the tube onto a body of water, such as a bay or down a streamor river, which floats the tube and acts as the support cradle), havingan internal vacuum as a result of both ends being swaged close, can thenbe cut up into desired lengths for use (or for sizing and/or grainrefinement since the ends are appropriately capped) or they can remainunchanged for float shipment to any desired shoreline location on earthby bundling into appropriate rafts. Such lengths can then be extendedinland (by means of bag rollers and use of the already laid pipe orpipes as a rail line) for end cutoff and weld or other attachment asmile-long lengths. The savings in transportation costs and decreasedwelding for pipeline fabrication is readily apparent.

It is a purpose of this invention to improve the invention of the Maxim(British Pat. No. 22,708 and that of Daubersy and Schlemmer (U.S. Pat.No. 2,940,143) by application thereto of Method 3 (a vacuum internal tothe tube being cast) or Method 4 (a positive external pressure exteriorto the tube and the exit orifice or at the entrance of the centrifuge)and combinations of Methods 3 and 4 In the Maxim process, as improved bythe foregoing means, a static (not axially flowing) centrifuged cylinderof liquid mold material has its interior diameter (adjacent to the exitorifice annular weir) substantially equal to the [.D. of the exitorifice of the centrifuge. No liquid mold material overflows the exitorifice weir except the dragout that naturally occurs with the Maximprocess. Small additions of liquid mold material are added to the systemby any convenient means so as to continually make up the liquid leveland compensate for any losses due to dragout, vaporization, etc. Theapplication of Methods 3 and/or 4, as taught in this invnetion'sdisclosure, may be utilized to decrease the CD. of the semisolidifiedtube (being cast) to a slight extent, or to its greatest possibleextent, or to any in-between extent as desired. Due to the Maxim processnot having available an exciting volume of liquid mold material whichcan be restricted to build up an aiding back pressure by the restrictionto flow methods of l and 2, the

present invention must depend to a slight or a large extent (dependingon the amount of application of the Methods of 3 and/or 4) on thediametrical shrinkage of the tube OD. as it cools to the desired exittemperature. The exiting rate of the tube is controlled, as in the Maximprocess, so that the CD. of the tube thermally shrinks to a less valuethan the ID. of the exit orifice considerably prior to passage throughthe annular exit orifice in order to preclude jamming.

The Methods of 3 and/or 4 are also applied as an improvement to theprocess of Daubersy and Schlemmer as a positive and practical means ofreducing the CD. of the tube (being cast) to one which is equal-to orless-than the exit orifice ID. The amount of application of Methods 3and/or 4 extends from the minimum to the maximum range as desired. Bythis means, small amounts of liquid mold material (normally less than 5percent of the throughput weight of the molten metal being cast to tubeon a timed basis) are continuously circulated through the system inorder to maintain a lubricating flow of liquid mold material between theoutside surface of the tube and the face of the annular exit orifice ofthe centrifuge. I also apply, as an improvement to the Daubersy andMaxim processes, the means shown herein for the positive extraction ofthe centrifugally cast tube so that a controlled rate of output of thecentrifugally cast tube can be effected and thus preclude the dangerofjamming the tube into the exit orifice of the casting machines. I alsoapply, to the teachings of Maxim and Daubersy, the methods of vacuumsealing at the exiting end (as by continuous collapse-deformation of theexiting tube to items of longitudinal structure or by intermittentvacuum seal closures at specific intervals of length of the tube) sothat the Methods of 3 and/or 4 can be effectively applied to thesesolder processes.

By the foregoing means (the vacuum sealing of the starting end andclosure sealing of the exiting tube by collapsing to a solid shape orsection and the application of Methods 3 and/or 4 of my invention) theMaxim and Daubersy processes are improved upon to the point where veryhigh rates of casting output can be obtained and the product limitationsof the Maxim process (as expressed by the formula 1, D=T, which is givenas an example for the system of liquid lead mold and a lowcarbon,low-alloy steel being cast thereon to tube) are removed.

My continuous centrifugal process not only produces a wide range oftubular products for use as such but it produces this variety of tube atsuch high rates of output (on a hundreds of tons per hour basis) thatthe tube can be economically and very advantageously used as a basicitem for the production of other items of longitudinal structure. It istherefore a bona fide continuous casting process that is highlycompetitive when compared to the current continuous casting of solidbillets and slabs. More than this, the collapse deformation of suchcontinuously cast tube (as a basic starting item of production) intoother longitudinal structural shapes can be readily and much moreeconomically done than by current techniques and this can beaccomplished by the use of very light mills (as light rolling mills) andwith very few passes. Capital investment is thus greatly reduced andthus augments the other economies of the process.

The foregoing advantages apply also to the Maxim and Daubersy processesonce they have been improved by application of the teachings of thisinvention.

OBJECTS OF THE INVENTION lt is an object of this invention tocontinuously centrifugally cast metal tube on a centrifuged and axiallyflowing layer of liquid mold material, consisting of molten lead or tinand alloys thereof, wherein the amount ofliquid mold material flowingthrough the system is equal to or greater than five percent by weight ofthe molten metal being cast to tube in the same time interval.

It is a further object of this invention to maintain the CD. of thesolidifying metal tube equal-to or less-than the diameter of the exitorifice of the centrifugal casting machine so as to permit a rapid butcontrolled egress of the cast tube without danger ofjamming at the exitorifice.

Another object of the invention is to utilize a vacuum seal at theentrance or starting end of the centrifugal casting machine for purposesto be subsequently noted.

Another object of the invention is to continuously collapse the tube toa longitudinal structural solid shape so as to form a vacuum tight sealfor the tube at the exiting end.

Still another object of the invention is to collapse a limited portionof the tube, as it exits from the machine, to form vacuum tight closuresat specified intervals along the length of the tube.

Another object is to cut off such lengths of tube at the midlength ofthe closure so as to maintain the integrity of the vacuum internal tothe tube and to obtain long useable lengths oftube having such closuresat both ends thereof.

A further object of the invention is to introduce a vacuum internal tothe tube, as it is being cast, as a Method of reducing the CD. of themolten metal tube to a diameter that is lessthan that which would resultfrom normal shrinkage due to the Archimedes principle. (Method 3).

A still further object is to maintain a positive pressure (above theambient atmospheric pressure) of inert or reducing gas, external to thetube, at the exit end of the casting machine asla Method of reducing theOD. of the molten metal tube to a diameter that is less-than that whichwould result from nor mal sinkage clue to the Archimedes principle(Method 4).

An alternative object is to maintain a positive pressure of inert orreducing gas at the entrance end of the centrifugal. caster, andexterior to the vacuum seal at that end as an alternate Method ofreducing the CD. of the molten metal tube to a diameter that is lessthan that which would result from normal sinkage due to the Archimedesprinciple.

An additional object is to utilize the methods of a vacuum internal tothe tube and a positive pressure external to the tube in a desiredcombination for the purpose of reducing the CD. of the solidifying tubeto a less value than would result by normal sinkage due to theArchimedes principle.

A still further object is to decrease the exit aperature between thetube OD. and the exit orifice l.D. so that a sufficient back pressuremay be built up within the liquid mold material to maintain the CD. ofthe molten metal tube within the caster equal-t or less-than the ].D. ofthe exit orifice (Method 1).

A still further object of the invention is to utilize all com. binationsand permutations solidifying the Methods (listed herein as l, 2, 3, and4) to maintain'the CD. of the moltenmetal tube within the caster(centrifugal casting machine) equal-to or iess'than the ID. of the exitorifice.

Another object of the invention is to utilize an extended: hot-zone atthe starting end of the caster in order to accentu ate the effects ofgravity segregation to obtain a useful result such as a lower carbonsurface on steel sheet for use in the au v tomotive industry.

Another object of the invention is to introduce a small percentage ofliquid mold material into the bottom of an annular molten steel trough,at the starting end of a centrifuge, so that the molten steel beingpoured into the trough preheats the restricted amount of liquid moldmaterial to an elevated nonchilling temperature which efi'ects aninitial hot-zone (for leveling or accentuated gravity segregation) priorto the molten steel coming into contact with the major and colder amountof liquid mold material further down the bore of the centrifuge.

It is a further object of this invention to provide acontinuouscentrifugal casting machine having no lip or weir at the exit end andwhich uses a liquid lining (denser than the metal being cast) to floatthe cast tube out of the bore.

A further object of the invention is to utilize a liquid mold materialof lead, tin, and alloys of lead and tin for the purposes of floatingthe cast tube out of the bore of the centrifugal tube caster.

Another object of the invention is to improve the Maxim process (BritishPat. No. 22,709 by application of the novel Methods herein disclosed.

Still another object of this invention is to improve the process ofDaubersy and Schlemmer (U.S. Pat. No. 2,940,143) by application of thenovel methods herein disclosed.

A further object of this invention is to so increase the casting rateand versatility of continuous centrifugal tube casting machines,utilizing a liquid mold, that the tube product can be used as a basiccontinuously cast item for economical conversion into other items ofstructure on a continuous or noncontinuous basis.

SPECIFIC PROBLEMS AND ADDITIONAL OBJECTS Gravity Segregation One of thelimitations encountered in centrifugal casting concerns the centrifugingof denser constituents towards the outside surface (and, conversely,lighter constituents towards the interior surface) by the high G"centrifugal forces. Under normal, fairly rapid solidification this is noproblem but it is sufficiently severe in some alloy systems as toobviate or limit the use of centrifugal casting. The variation ofcomposition from the interior to the exterior surface of a centrifugalcasting is termed gravity segregation" and has been considered as eithera limitation or a nuisance by centrifugal casters.

It is a purpose of this invention, and one of its features, to enhanceand utilize gravity segregation to a useful purpose.

The specific method of accomplishing or enhancing gravity segregation toeffect a useful purpose is to introduce as entended-hot-zone at thestarting end of the continuous centrifu' gal casting system hereindisclosed. The Maxim process has a hot zone at the starting end of thecaster for the purpose of preventing a knobby surface (to enhance theleveling or smoothing action) and another invention, US. Pat. 2,754,559issued to Fromson in 1956, utilizes an initial hot-zone to enhancelayering or smooth spreading out of the molten metal to be solidified ontop of a flat liquid mold of lead. in the present process, the hot-zoneis appreciably extended, (where desired to enhance gravity segregationand only in this instance is the hot-zone so extended, beyond thatrequired for effective leveling or layering of the molten steel) so thatsegregation will be-emphasized and can be utilized in a very worthwhilemanner as will be explained in detail later on.

Automotive sheet steel (used for the exterior body covering) is normallymade from rimmed-stcel ingots even though it would be considerablycheaper, if the desired properties were present, to utilize continuouslycast slabs or billets instead of remaining with the old ingot process.The reason for this is that rimming-steelexhibits a vigorous boilingaction onlpouring into the ingot mold and this creates a scrubbingaction at the solidifying surface of the ingot. The result is thatrimmedsteel ingotsihavea fine grained exterior layer of fairly lowcarbon contenLWhen such ingots are rolled, the surface of the sheet issmoother and takes a better polish than:steel made by other processes.It also has a better deep drawing qualities. The spattcring (whichcreates a rim on the ingot mold and is the basis for the termrimmedsteel) caused by the release of gases, with resultant vigorousboiling action, is the main reason that rimmed steel cannot beeffectively cast by current continuous casting processes.

Rimming-steel can be cast in the centrifugal process using a mold havinga fairly large diameter (as 3 feet) since any spattering merely ends upon the opposite interior surface of the tube. The scrubbing action isabsent, however, since the released gases are directed inwardly by thecentrifugal forces. C'entrifugally ,cast steel does, however, have therequired density since it is pressure cast under optimum conditions.

If, however, an extended-hot-zone is used, either with,

rimming steel or with semior fully-killed low carbon steel, the deltaferrite (essentially pure iron) solidifys first, and being solid anddenser than the balance of the molten metal, centrifuges to the exteriorsurface. The resultant centrifugally cast tube is characterized byhaving an exterior layer of dense, fine grained, low-carbon steel. Sucha tube can be collapsed to a plate and rollwelded on its interiorcontiguous surfaces to yield a product capable of being rolled to sheetstock which exhibits all of the properties (smooth surface, highpolishability, and deep drawing characteristics) required of automotivesheet stock. Such a tube can also be slit longitudinally and flattenedtoplate stock, by prior art processes, and rolled to sheet havingthedesired properties on one, the tubes exterior, surface.

It can be appreciated that such automotive sheet stock can also beproduced from batch-type centrifugally cast cylinders of steel by theexpedient of an extended (slow) cooling action using preheated or lowheat conductivity molds of a solid wall nature.

The extended-hot-zone is basically a means of slowing the solidificationrate over a specific temperature range. With low-carbon steel this rangecoincides with the delta-ferrite region of the iron-carbon phase diagramwhich encompasses the temperature range of about I ,500 to l,475 C.

The extended-hot-zone (slowed solidification range) can, by intentionalvarying of the length of the hot-zone or utilizing higher G forcescreate a wide variation of surface properties in collapse-formed sheetproducts made from such tube. Ordinarily, the extended-hot-zone is usedonly where an end product of uniquely advantageous properties is created(as automotive sheet stock). The hot-zone is restricted to thatnecessary for leveling or smoothing of the molten steel or other metallayer under all other conditions. This is especially true where the tubeis to be longitudinally collapsed -formed to a structural item (asl-beam or railroad rails) where a lower carbon surface could result in aloss of fatigue resistance.

Other alloys can be advantageously processed by the technique of usingan extended-hot-zone. Cast iron pipe continuously centrifugally castfrom gray or nodular irons can be produces with a gradient metallurgicalstructure (from the exterior to interior surface of the pipe) of varyingcarbon content which exhibit advantageous properties under certainconditions of use.

Lead-Tin Alloy Liquid Mold Whereas the Maxim process utilizes lead andsome alloys thereof for a liquid mold material, as does my processherein disclosed, it is an object of this invention to improve theliquid mold material by additions of tin to the lead and the use oflead-tin alloys and tin as liquid mold materials is claimed when used inconjunction with this invention.

Tin is particularly used as an addition to the liquid lead mold materialwhen it is desired to retain the exterior lead film on the tube as acorrosion resistant barrier both for collapseformed items of structureand, in particular, for use in pipelines since tin greatly increases theadhesion of lead to other metal surfaces. Both lead, tin, and lead-tinalloys are very corrosion resistant and have been historically used forthis purpose. Other advantages of tin additions to the lead includelowering the melting point and raising the boiling point beyond that oflead alone and this extends the usable liquid range of the liquid moldsystem. Tin additions also increase the fluidity and heat conductivityof the liquid mold and, more important, tend to suppress the vaporizingtendency of lead at elevated temperatures. It thus helps to prevent leadlosses, due to vaporization, and reduces the danger of toxic lead vaporsescaping from the system. The foregoing advantages outweigh the extracost incurred by tin additions to the liquid lead mold.

It can be realized that, by continuously casting metal tube on anaxially flowing ring of liquid mold material (the ID. of which ismaintained equal-to or less-than the ID. of the exit orifice of thecentrifuge) the solidified or semisolidified tube will float out of thebore on the axially flowing liquid mold material without danger ofjamming in the exit orifice. More than this, the wall thickness todiameter restrictions of the tube output are largely obviated and muchsmaller diameter tubing can be continuously produced.

The liquid mold material, used in conjunction with the continuouscentrifugal casting systems herein disclosed, embodies the followingcharacteristics: (1 has a solidification temperature lower than that ofthe material being cast; (2) is substantially immiscible with andnonreactive to the molten material being cast (except where alloying isdesired for a corrosion preventive surface coating such as tin on iron);(3) has a boiling point which is substantially higher than the meltingpoint of the material being cast under the rotational forces involved(high G rotation suppresses the boiling tendency); and (4) has a densitywhich is greater than the material being cast to tube. Liquid lead andlead-tin alloys are generally used in the casting of light metals suchas titanium, aluminum, magnesium, etc.

The novel features which are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe description when read in connection with the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a graphical representation of the change inspecific volume ofa solidifying and cooling steel;

FIG. 2 is a graphical representation of the formulas l and 2respectively which show the limitations of product output of liquid moldcentrifugal tube casting machines which depend on diametrical shrinkageof the solidified tube to accomplish extraction thereof;

FIG. 3 is a diagram of a unit volume section of tubing wall, in the formof a truncated wedge with radial sides, used in computing the pressuredifferential required for counterbalancing the expansion effect ofcentrifugal force on rotating tubing being cast by my process;

FIG. 4 is a partial sectional view of a simplified centrifugal,liquid-mold continuous casting machine wherein no exit orifice lip(reduced diameter annular orifice weir) is used;

FIG. 5 is a more sophisticated axial sectional view of a liquid-moldcontinuous centrifugal casting machine adapted to the floating of thetube out of the bore;

FIG. 6 is an axial sectional view of one embodiment of this inventiondepicting vacuum sealing means at the entrance (pouring) end of thecentrifuge and a means of vacuum sealing the tube subsequent to the exitend;

FIGS. 6A, 6B and 6C are partial axial sectional views depicting otherembodiments of the entrance end vacuum sealing means;

FIG. 7 is an axial sectional view of an embodiment of the exit end of acentrifugal casting machine which depicts means of enclosure to effect apositive pressure (above ambient) external to the exiting tube;

FIG. 8, 8A and 8B are partial axial sectional views depicting variousmeans of layering the molten metal onto the liquid mold material in asmooth continuous manner.

DETAILED DESCRIPTION Referring now to the drawings in detail, and inparticular to FIG. 1 (redrawn from Wulffs Metallurgy for Engineers), Ihave shown, by way of example, that a centrifugally cast mild steel tubewill experience a diametrical shrinkage of about 2 percent in coolingfrom the solidification temperature of about 1 ,500 C. to a temperaturejust above that of the melting point of a liquid lead mold material or330 C. I have also shown that the diametrical shrinkage of acentrifugally cast mild steel tube in cooling from 1,500" C. down to 700C. is about 1.53 percent.

By using these percent shrinkage values and the densities of the axiallyflowing centrifuged molten tube of mild steel and the liquid lead moldat temperatures involved, I have derived formulas l and 2, given earlierwhich illustrate the minimum diameter of a mild steel tube for any givenwall thickness, in order to satisfy the displacement requirements of theArchimedes principle and the diametrical contraction requirements forwithdrawal of the tube from the exit orifice of the centrifugal castingmachine where such tube shrinkage is the means by which such exit isaccomplished.

The limitations of formulas l and 2 (D=65T and D=T respectively) aregraphically illustrated in FIG. 2 wherein, for any wall thickness ofmild steel being centrifugally cast to tube on a liquid mold of lead,the tube diameter, necessary to permit sufficient contraction of thetube so that it can just escape out of the systems exit orifice, canreadily be determined. It should be realized that these are merelyexamples formulas and graphical figures which are applicable to thecontinuous centrifugal casting of a mild steel tube on a liquid leadmold. Similar formulas and graphs can readily be derived for othersystems of casting materials (as aluminum, copper, nickel, etc.) whenused in conjunction with other liquid mold materials (as lead, tin, andleadtin alloys).

Reference is now made to FIG. 4 which is an axial cross-sectional viewof a simplified version of a continuous centrifugal tube caster (castingmachine) or centrifuge utilizing a liquid mold and having an exitorifice diameter which is equal to or greater than the CD. of the tubebeing cast. In FIG. 4 the centrifugal caster is rotatable about its axis1 by means of suitable trunnions, and drive mechanisms not shown. At theentrance orifice 2 a liquid mold material 3 is poured upon the rotatingannular refractory and thermally insulating part 4 of the centrifuge viaspout 5. At the same time, the molten metal 6, to be cast to tube, ispoured onto the refractory part 4 of the centrifuge by way of spout 7.The refractory part 4 of the centrifuge extends to a point 8 (towardsthe exit end 9) so as to form a hot-zone 10 wherein solidification ofthe metal tube is retarded and where the molten metal 6 and the liquidmold material 3 have time to layer into over-and-underlaying cylindricalshells in the liquid state. The refractory part 4 is enclosed in astructural shell 11 which supports the refractory part 4 and thenextends to the exit end 9 as the solid wall 12 of the centrifuge. Thesolid wall 12 is cooled on its exterior surface by multiple peripherallyarranged jets of Water (not shown) or other cooling material so as toremove heat from the molten metal 6 through the liquid mold lining 13andsolidify the molten metal to a solid tube 14. The solidified tube 14continues out of the centrifuge into an axially aligned and rotatingcradle (not shown) and is intermittently cut off to desired lengths byany desired mechanism such as that shown in the Maxim patent. The liquidmold material 3 cascades at 15 from the annular exit end 9 of thecentrifuge into an annular trough (not shown) such as that used in US.Pat. No. 2,866,703 issued to Gross in 1958 and wherein an axiallyflowing molten metal effluent is spun out of the exit end of acentrifuge into an annular catch basin. The liquid mold material 3 isthen recirculated back to the pouring spout 5 by any convenient meanssuch as that of U.S. Pat. No. 2,6l7,l48 issued to Ryan in 1952 andwherein a metallic liquid mold material is recirculated from the exitend of a casting machine back to the entrance end via suitable heatexchangers (coolers) and a suitable pump.

It should be noted that the continuous centrifugal tube casting machineof FIG. 4 utilizes a long bore so as to accentuate the shearing action(resistance flow) in the liquid mold material. A positive pulloutmechanism of any type (such as that used in British Pat. No. 22,708issued to Maxim) is used to control the rate of exit of the cast tube sothat it is sufficiently solidified prior to exiting from the end 9. Therefractory part 4 of the centrifuge is preferably made of pyrolyticboron nitride or pyrolytic graphite with the C" planes (the plane of lowheat conductivity) being perpendicular to the axis 1 of the bore and the"A plane (the plane of greatest heat conductivity) being parallel to theaxis ofthe bore. In this manner, the inside (I.D.) of the hot-zone 10 isat a high and uniform heat that prevents solidification in that area. Bygreatly extending such a hot zone, an extended-hot-zone results whichpermits the accentuation of gravity segregation to a useful extent.

This system has the virtue of extreme simplicity; however, due to thehigh G forces involved the liquid mold material has a higher excitingflow than the cast tube with its controlled pullout. This flowdifferential can cause wrinkling (shirt-sleeving) of the tube surface atthe point of incipient solidification and this surface roughness anchorsthe liquid mold material and results in excessive dragout.

FIG. 5 is illustrative of a more sophisticated system for the continuouscentrifugal casting of metal tube on an axially flowing lining of liquidmold material. A criterion of the apparatus of FIG. 5 is that the CD. ofthe molten metal tube (within the centrifuge) be equal to or less thanthe exit orifice I.D. In FIG. 5, the molten metal 6 pours into anannular trough 16 which is similar to the annular distributing chamberused by Stravs and Jager in US. Pat. No. 777,559 of 1904 and serves totake up the impact of the inpouring molted metal 6 and to evenlydistribute the molten metal, via the refractory annular shelf 17, as amolten cylindrical tube within the bore of the centrifuge. Therefractory part 4 of the centrifuge is extended towards the exit end 9,as shown, so as to form a hotzone 10 whereon the cylinder of molten tubemetal 6 becomes leveled or layered into a smooth cylindrical tube 26 ontop ofa thin cylindrical layer 18 of hot liquid mold material.

The liquid mold material 3 is poured into an annular sump l9 and moves(via multiplicity of longitudinal holes 20 peripherally spaced aroundthe base of the refractory part 4) downstream in the centrifugal castervia the main series of flow-holes 20 to the maine exit 21 what the mainpart of the cooler liquid mold material flows into a heat extractingring 22 of liquid mold material which both supports and solidified thering of molten metal to an exiting soli-d tube 14. The ring 22 of liquidmold material may be in contact with a finned wall portion of thecentrifuge, which is cooled by sprays of water or other coolant fromnozzles 81, supplied via piping 82 and a pump P.

Upstream from the main exit 21 of the liquid mold material is anotherseries of annular liquid mold flow-holes 23 via which a restricted(quite small) amount of liquid mold material forms a thin lining 18 ofvery hot liquid mold material which extends downstream for the length ofthe hot-zone l0 and permits rapid and effective cylindrical layering andleveling of the molten'6 and liquid 3 materials. The cylindricallylayered ring 26 of molten metal substantially solidifies to a solid tube14 on the ring 22 of heat-conducting liquid mold material which flowsaxially down the bore of the centrifugal tube caster towards the exitend 9 and becomes a thin ring 24 of restricted flow (in accordance withMethod 1 for creating a back pressure on the liquid mold material 3) asit passes over the exit orifice weir 25 having an axially extendedsurface area, adjacent to the periphery of the solidified tube 14, whichcreates a line pressure drop along its length (in accordance with Method2 detailed in this disclosure) which accentuates the back pressure onthe liquid mold material 3 to the extent that the CD. of the moltenmetal tube is maintained equal to or less than the ID. of the exitorifice weir. The rotating solid tube 14 exits axially from thecentrifuge for cutoff, seal crimping, or continuous collapse deformationas desired while the liquid mold material 22 spins offas a tangentialstream 15 into a suitable annular catch-ring 83 and is recirculated byconventional means not shown. These means, along the the rotationalmechanisms and spray cooling method, are indicated but are not detailedsince they are a part of the prior art and well understood by thoseversed in such techniques. Here also, the hot zone as at 10 may beextended in length so that slow cooling of the molten metal can beaccomplished. In this manner, when desired, accentuated gravitysegregation results (as delta ferrite being centrifuged towards theoutside surface of a mild steel tube which is later to be converted toautomotive sheet steel).

In FIG. 5, the ring of axially flowing and heat extracting liquid moldmaterial 22 (between the downstream end of the hot zone 10 and theupstream end of the exit orifice weir 25) has a preferred thicknessapproximating the thickness of the tube wall being solidified thereon.In this manner, the molten and solidified metal 26 and 14 of the tubeflows axially in approximate synchronization with the axial flow of theliquid mold material and this results in a smoother exterior surface onthe solid tube and less retention of the liquid mold material thereon.

FIG. 6 is illustrative of a vacuum seal means at the entrance end 2 ofthe liquid mold continuous centrifugal tube casting machine wherein asolid nonrotating disc 30 has it periphery 31 immersed into the liquidmold material 3 which is con-. tained in the annular rotating trough l9.Passing through and vacuum sealed to the nonrotating end plate 30 arethe liquid mold circuit 5, the molten metal conduit 7, a dry inert gaspurge tube 32, and a vacuum suction outlet 33. The purge solid 32 (orother sealed entrance conduit) may be used as a plasma torch entrancefor the purpose of heating up the refractory part 4 prior to startup. Inthis instance, the inert gas (as helium, argon, nitrogen, etc.) from theplasma torch also acts as an initial purge of the centrifuge cavity andthe torch melts down the starter blank which has solidified within thebore of the centrifuge from the prior shut down operation. The suctiontube 33 is fairly large and connects to a vacuum pumping system (notshown) so that the interior cavity of the centrifuge can be continuouslypumped down to any desired vacuum.

Exterior to the exit end 9 of the centrifugal casting machine is a setof opposed forging rolls 34 and 35 which travel axially and insynchronism with exiting tube 14. At the same axial location and atright angles to the plane between the axis of the forging rolls (34 and35) are two opposed banks of burners (as not shown plasma torches) whichmaintain the heat of the exiting tube 14, or bring it to a desired forgewelding temperature. These forging rolls 34 and 35 move synchronouslyand axially along with the hot tube and gradually come together withsufficient force to collapse a small portion of the tube (as a 2 footlength) to a solid round having a forge welded interior 36 which isvacuum tight. Such collapsed sections of the tube can be as far apart asdesired (as every 300 feet of solid tube length) and provide the vacuumseal to the tube at the exit end of the centrifugal caster. Further on,and after another seal has been so forge-closed, the solid section 36can be cutoff at its midlength 37 for removal of the discrete length ofthe vacuum sealed sausagelike tube lengths, for use as previouslydescribed. It can be appreciated that other conventional means, asswaging flat-crimping, etc. can be used to form the discrete collapsedsection for vacuum closure, beyond the exit end 9, of the hot tube.Also, the axial travel of the sealing rolls (34 and 35) can be extended(as to 300+ feet) so that they act as pullout grips for the tube socast.

FIG. 6A is a partial sectional axial view of another configuration ofthe entrance end 2 vacuum seal means wherein the stationary seal disc 30is peripherally immersed in an annular trough 40 of a low melting liquidmetal such as Woods Metal or molten tin. It has the advantage ofpermitting the seal to be at a lower temperature and obviates oxidationlosses of the seal fluid. In this case, both the molten metal and theliquid mold material are subjected to the internal vacuum at theentrance end 2.

FIG. 6B is representative of another such configuration wherein theannular seal trough 40 is intermediate between the molten metal troughl6 and the liquid mold material trough 19. By this means, the liquidmold material is not subject to the internal vacuum but to the ambientatmospheric pressure and this helps to raise the level (decrease theO.D. of the molten tube 26) of the liquid mold material within the boreof the caster.

FIG. 6C is yet another variation of the vacuum seal means at theentrance end 2 wherein the method of FIG. 6B is further enhanced by useof another end plate 41, exterior to the end plate 30, which isperipherally immersed into an annular rotating trough of liquid sealingmetal 42. This system permits the liquid mold material 3 in annulartrough 19 to be pressurized via inert gas tube 38 while this interiorcavity of the centrifuge is subjected to vacuum. The system of FIG. 6Cis even more effective in reducing the O.D. of the molten metal tube 26to the desired size.

FIG. 7 is illustrative of a means for applying a positive pressure ofinert gas 50 to the outside of the solidified tube 14 at the exit end 9of the continuous centrifugal tube caster. The inert gas 50 isintroduced into the end closure 51 via the high pressure gas tube 52 andthe pressurized gas 50 acts on the liquid mold material 3 at the pointof tangential spinoff so as to produce a greater than normal backpressure on the liquid mold lining 22 within the centrifuge. This backpressure (Method 4) causes the heat extracting ring of liquid moldmaterial at 22 to push inwardly and decrease the OD. of the molten metaltube to any desired limit.

The end closure 51 is sealed at the annular area 53 (exterior to theexit end 9 of the centrifugal tube caster) by means of an iris ring ofcarbon or graphite blocks 54 which are contained within the annularholding-rings 55. An annular pressure cavity 57 is behind the irisblocks 54 so that, by pressurizing this annular cavity 57 by means ofthe high pressure inert gas line 58, the iris blocks 54 are forcedagainst the O.D. area 53 of the centrifuge to form a pressure seal.Alternately, the seal at the area 53 can be of the liquid metal type asdesignated by trough 40 of FIG. 6A.

A similar inert gas pressure seal exists at area 60 on the opposite sideof the end-closure closure 51 so as to prevent undue gas leakage aroundthe tube periphery. This iris of carbon ploughs or blocks 61 also act asscrapers to remove any excess liquid mold material from the periphery ofthe tube. Alternately, a carbon iris block 62 can be used which has amultiplicity of small radial holes 63 leading from the annular pressurecavity 64 to the ID. of the blocks 62 are the area 60. Passage of highpressure inert gas (as nitrogen) through the holes 63 onto the peripheryof the tube 14 at area 60 causes a gas bearing action which wipes backany excess liquid mold material into the closure 51 and, at the sametime, maintains the desired inner gas pressure therein. As a stillfurther alternate, the pressure cavity 64 may be pressurized withrelatively cool liquid mold material 3 so that a liquid bearing seal isformed. This alternate would only be used where a maximum amount ofliquid mold material was desired as an exterior coating to the tube soproduced.

Referring to FIG. 8, this partial axial sectional view of the entranceend 2 of the centrifugal continuous tube caster illustrates a simplifiedmeans of sluicing the molten tube metal 6 onto the 1D. surface of theaxially flowing ring 22 of liquid mold material 3. In FIG. 8, theperipheral flow-holes 20 for the liquid mold material 3 terminatedownstream at a point 27 and the refractory part 4 continues downstreamand tapers to an annular feather edge at point 28. At point 28 the axialflowing annular rings of molten tube metal 26 and of liquid moldmaterial 22 cone into heat exchange contact with a layered laminar flow.The shelf 17 of the refractory part 4 acts as a hot zone for layeringand leveling of the molten metal 26. This is the simplest technique, butnot the preferred one, for introducing the molten metal layer 26 ontothe liquid mold layer 22.

FIG. 8A represents an improvement of the method for sluicing the moltenring of axially flowing metal 26 onto the axially flowing ring of coolliquid mold material 22 via an interposed thin ring 18 of axiallyflowing hot liquid mold material which is introduced onto the shelf 17of the refractory part 4 by way of small inclined flow-holes 23 toproduce a hot zone or extended hot zone 10 as desired.

The preferred technique for producing a hot or extended hot zone 10 andfor bringing the axially flowing annular streams of molten metal and hotand cool liquid mold materials into laminar contact is illustrated inFIG. 8B. In this technique, an annular trough I6 is filled with a smallflow of liquid mold material 3 by way of the small ducts 23 which leadfrom the liquid mold trough 19 to the bottom of the molten metal trough16 from whence it flows internal to the ledge I7 of the refractory part4 as a hot relatively thin lining which supports the molten metal ring26, The molten metal 6 pours onto the surface of the liquid moldmaterial which fills the trough l6, and heats the liquid mold materialto a temperature above the melting point of the tube metal. The annulartrough 16 serves the purpose of decreasing the impact of the moltenmetal input 6 and of creating a very effective layering and levelingzone even prior to the downstream hot-zone represented by the relativelythin hot liquid mold lining 18. At the downstream sharp edge 28 of therefractory part 4, the hot liquid mold lining I8 continues downstream.for a short distance and acts as a buffer between the axially flowingcool liquid mold ring 22 and the molten tube metal 26 and prevents toorapid chilling of the metal tube. It is preferred that all three annularrings (the molten metal ring 26, the hot liquid mold l8, and the coolerliquid mold 22) have an approximately synchronized axial flow rate atthe point 29 where solidification of the molten tube metal begins.

All of the systems illustrated in FIGS. 8 to 88 can be used inconjunction with the entrance end vacuum seal means of FIGS. 6 to 6C.

Rejuvenation f the Liquid Mold Material Regardless of the use ofinternal and external inert atmospheres, the liquid mold (whether oflead, tin, or lead-tin alloys) will gradually build up an oxide contentwhich, being lighter than the liquid mold material, will centrifuge tothe LD. surface of the liquid mold and adhere to the CD. of the metaltube being cast. Normally, this concentration is not large enough tocause problems but, at heavier concentrations, it can cause excessivedragout of the liquid mold material and, in extreme cases, clogging ofthe conduits and flow-holes of the system. This can be corrected eitherby continuous or occasional passage of the liquid mold material througha bath of molten cyanides (asthose of sodium, potassium or barium ormixtures thereof). By such treatment, an oxidation-reduction reactiontakes place that produces a reduced liquid mold material that iscompletely rejuvenated (oxide free).

Start and Stop Procedures ln stopping the process, a clutch system, notshown, is released that stops the pullout of the tube 14, as in figs.5,6,7, from the exit orifice 9 of the centrifuge while, at the sametime, permitting the tube to continue rotating along the withcentrifuge. Coincidental with stopping the axial pullout of the tube,the input of molten metal 6 is terminated. The centrifuge is allowed torotate until the ring of molten metal 26 on the inside of the centrifugehas solidified. Normally the centrifuge rotation is continued and theliquid mold material circulation is maintained (but bypassed through anexternal heating system to keep it in the liquid state) until the nextstartup.

Where complete shutdown is made (as for repairs), the solidified tubewithin the centrifuge is withdrawn by re-applying the pullout clutch(not shown), the input of liquid mold material 3 is terminated, and thecentrifuge is then braked to a stop. The liquid mold material is allowedto cascade out of the system via the annular catch-ring and drained (bymeans of suitably located drain-plugs) into a suitable holding tankwhere it can be heated and maintained as a liquid until further use.Said holding tank can also contain the molten eyanides which float onthe surface of the liquid mold material and removes any accumulatedoxides as well as shielding the liquid mold material from contact withthe air, during storage thereof.

On startup (under normal nondrained conditions), the internal vacuum isreleased by input of a dry inert gas via the purge tube 32. An inert gasplasma torch is then introduced through any convenient and resealableaperture (not shown) in the stationary end plate 30. The torch flame isdirected onto the solidified tube overlaying the hot zone 10 and theannular trough 16 until the solid tube again becomes the molten metallayer 26. The torch is then quickly removed, the required vacuum redrawnon the tubes interior, and pullout of the tube 14 is recommenced.Coincident with the tube pull out, the input of molten metal 6 isstarted up.

Where complete shutdown is used, the start up (previously solidifiedinternal tube) blank is reintroduced into the bore of the centrifuge,rotation is started, and the starting end over the hot zone 10 andtrough 16 is preheated by the plasma torch. The flow of liquid moldmaterial is restarted. The plasmatorch then melts down the hot but stillsolid tube at the starting end, the seal trough (s) is filled, theinternal vacuum is reestablished and pullout is recommenced along withinput of molten metal 6.

It should be noted that plasma torch can be made integral (normally notremovable) part of the end-seal since such torches are usually watercooled and not subject to heat damage.

It should be further noted that the starting blank will always have theexternal end forge-closed at a point 36 in any case where an internalvacuum (Method 3) is utilized in the various systems of this inventionsdisclosure.

Grain Refinement In general, centrifugally cast metal tube ischaracterized by columnar grains extending radially inwards from theexterior surface. Such grain type is an advantage where the tube is usedat elevated temperatures and pressures since a coarse-grained structureinhibits creep deformation. However, for most purposes, a fine grainedmaterial is desired due to its more favorable mechanical properties.Where the tube is collapsed and roll-sized to structure, such grainrefinement can be accomplished due to the hot-working recrystallization.In the instance where the tube is to be used, as such (as for oil linepipe, etc.), grain refinement can be accomplished either during thecontinuous centrifugal casting process or subsequent to its cooling toroom temperature.

In the first instance (grain refinement coincident with tube casting), ashearing action can be set up between the external shell of alreadysolidified metal and the interior layer of still molten metal (asdownstream from point 29 of FIG. 8). This can be done by mechanical ormagnetic means and the layer of still molten metal can be either sloweddown or speeded-up rotationally so that the still molten metal has acircumferential speed that is different from that of the alreadysolidified exterior shell metal. ln this manner, the shearing action atthe solid-liquid interface destroys the columnar grain growth andcreates an equiaxed fine grained structure in the solid tube metal.

Such differential rotational speed between the solid exterior shell andthe inner still molten layer of metal can be caused by an interiorrefractory drum (of light, hollow construction and having an CD. whichis less-than the ID. of the molten metal wall 26) which rotates eitherfaster or slower than the centrifuge and is driven by a cooled shaftextending through the stationary vacuum-seal end plate 30. Rotatingskimmer blades can also be used. Such differential solid-liquidinterface shear can also be created by a rotating magnetic flux internalto the centrifuged tube by an adaption of the method of Pestel asdisclosed in US Pat. No. 2,963,758 of 1960.

Grain refinement of the tube metal once it has exited from the castingmachine can be accomplished by pulling the hot exiting tube through arotating sizing bell or by drawing the tube, in the cold state, throughnonrotating internal and/or external sizing dies which cold-work thetube metal while sizing it. Where discrete lengths of tube, having theends sealed by forged closures, are made, a high pressure aperture canbe made in one end and the tube length can be hydroforged as per theteachings of US. Pat. No. 2,931,744.

In both cases where cold working is done on the tube metal, grainrefreshment is accomplished by subsequent reheating to itsrecrystallization temperature.

It is a primary purpose of this invention to so increase the continuouscasting rates (especially for steels and cast irons) of the metals thatthe tubular output will be used as a basic item for the production oflongitudinal structural shapes by collapse deformation of such tube. Theprimary method of utilizing such tube would be to cut off discretelengths as the tube exits from the continuous casting machine and thento deform the tube to other structure while still hot or, alternatively,by cooling to room temperature and subsequently reheating the stockpiledtube to the forming temperature desired. Such collapse forming of tubeto a desired cross section (as l-beam plate, channel, etc. and rollingto final size, with accomplishment of roll-welding of the contiguousinner surfaces of the tube, requires that the interior surfaces of thetube be clean and, preferably, in a fluxed condition.

I propose to protect and flux the interior surfaces of the tubes byblowing a powdered material (anhydrous borax- Na B 07in particular) ontothe interior surface of the tube at a point where the temperature of thetube is less than the decomposition temperature of the flux and wellprevious to the point where the tube interior cools below the meltingtemperature of the flux. In this manner, the tube interior remainscoated with a thin layer of protective flux which is squeezed out duringany subsequent roll-welding of the interior and can be collected forreuse. Alternatively, the flux can be introduced into the tube interiorby spraying in the molten condition by any conventional means.

Alternatively, molten salts such as the higher melting and boilingalkali and alkaline earth chlorides and fluorides (or

2. The method defined in claim 1 wherein said diameter control iseffected by pressurizing said liquid lining within the mold so as toincrease its constricting pressure against said tubular body over thenormal buoying effect determined by the displacement of the tubematerials''a own weight of the liquid lining.
 3. The method defined inclaim 2 wherein said pressurizing is effected by providing said moldoutlet with an annular lip which is extended axially sufficiently tocreate a substantial frictional resistance to flow of said mold liningto counterbalance the weight of the molten casting material.
 4. Themethod defined in claim 2 wherein said pressurizing is effected byproviding said mold, having no lip, with an axially extended bore ofsufficient length to create a substantial frictional resistance to flowon said liquid lining.
 5. A method for continuously casting tubing of amaterial of predetermined specific gravity and melting temperature,comprising the following steps: rotating on a generally horizontal axis,an elongated tubular mold having an inlet at one end and an outlet atits other end; injecting into the inlet of the rotating mold a liquidsubstance of higher specific gravity and lower melting temperature thansaid casting material and causing said substance to acquire the form ofa cylindrical lining within said mold in response to the rotation of themold and resulting centrifugal force, and thereby providing acylindrical casting chamber within the mold; injecting said moltencasting material into said cylindrical casting chamber and causing it toassume the form of a cylindrical tube in response to the rotation ofsaid lining, and causing said tube of liquid casting material to becooled by said lining and to thereby be congealed to a state within therange including semisolid and solidified states; causing said lining toflow from the inlet to the outlet of the mold and to discharge from saidoutlet while simultaneously causing said tube to exit from the outletwhile enveloped in the outflowing lining of liquid substance; andcausing controlled exit of said tube and aiding in controlling thediameter of said tube to permit clearance through said outlet bysubjecting said tube to a gas-induced pressure differential while it isstill in a relatively plastic state.
 6. The method defined in claim 5wherein said aid to diameter control is effected by creating a vacuumwithin said tube.
 7. The method defined in claim 5 wherein said aid todiameter control is effected by creating a back pressure on the liquidlining by applying supra-atmospheric pressure to the exterior of thedischarging tube.
 8. The method defined in claim 5 wherein said aid todiameter control is effected by creating a pressure on the liquid liningby applying supra-atmospheric pressure to the liquid lining material atthe entrance end of the casting machine.
 9. A method for continuouslycasting tubing of a casting material of predetermined specific gravityand melting temperaTure, comprising the following steps: rotating on agenerally horizontal axis, a bulbous tubular mold having an inlet at oneend and an outlet at its other end; filling the bulbous cavity of therotating mold with a liquid substance of higher specific gravity andlower melting temperature than the casting material and causing saidsubstance to acquire the form of a cylindrical lining within said moldin response to the rotation of the mold and resultant centrifugal force,and thereby providing a cylindrical casting chamber within the moldhaving a liquid lining which is substantially equal to, but does notoverflow, the exit orifice weir; injecting said molten casting materialthrough said inlet into said cylindrical casting chamber and causing itto assume the form of a cylindrical tube in response to the rotation ofsaid lining, and causing said tube of liquid casting material to becooled by said lining and to thereby be congealed to a solidified state;causing controlled withdrawal of said tube from the exit orifice; andapplying a gas pressure differential between the inside and outside ofsaid tube in substantially its molten state to facilitate its withdrawalfrom said mold.
 10. The method defined in claim 9 wherein said pressuredifferential comprises a partial vacuum within said tube.
 11. The methoddefined in claim 9 wherein the tube within the mold is partiallysupported by a supra-atmospheric pressure exterior to the tube at theexit end of the casting machine.
 12. A method for continuously castingtubing of a material of predetermined specific gravity and meltingtemperature, comprising the following steps: rotating on a generallyhorizontal axis, an elongated tubular mold having an inlet at one endand an outlet at its other end; injecting into the inlet of the rotatingmold a liquid substance of higher specific gravity and lower meltingtemperature than said casting material and causing said substance toacquire the form of a cylindrical lining with said mold in response tothe rotation of the mold and resultant centrifugal force, and therebyproviding a cylindrical casting chamber within the mold; injecting saidmolten casting material into said cylindrical casting chamber andcausing it to assume the form of a cylindrical tube in response to therotation of said lining, and causing said tube of molten castingmaterial to be cooled by said lining and to thereby be congealed to astate within the range including semisolid and solidified states,creating a partial vacuum inside said tube sufficient to aidsubstantially in reducing its diameter for clearance through saidoutlet; causing controlled output of said tube; and closing said tube atselected positions along its length in order to seal said tube and tocreate a vacuum seal at the outgoing end of the tube.
 13. A method forcontinuously casting tubing of a casting material of predeterminedspecific gravity and melting temperature, comprising the followingsteps: rotating on a generally horizontal axis, an elongated tubularmold having an inlet at one end and an outlet at its other end;injecting into the inlet of the rotating mold, a liquid substance ofhigher specific gravity and lower melting temperature than said castingmaterial and causing said substance to acquire the form of a cylindricalsaid said mold in response to the rotation of the mold and resultantcentrifugal force, and thereby providing a cylindrical casting chamberwithin the mold; injecting said molten casting material through saidinlet into said cylindrical casting chamber and causing it to assume theform of a cylindrical tube in response to the rotation of the lining,and causing said tube of liquid casting material to be cooled by saidlining and to thereby be congealed to a state within the range includingsolidified and semisolidified states; causing said lining to flow fromthe inlet to the outlet of the mold and to discharge from said outletWhile simultaneously causing said tube to exit from the mold outletwhile enveloped in the outflowing lining of liquid substance; causingcontrolled output of said tube; maintaining an annular hot-zone in saidcasting chamber near said inlet and extending toward said outlet;bypassing the major proportion of said liquid mold lining substance in abypass flow externally around said hot-zone and then directing itinwardly to the diameter of said lining; and directing a ducted smallflow of said liquid mold lining substance diagonally forwardly andinwardly to said hot hot-zone from said bypass flow, to promote flow inthe hot zone.
 14. The tube-casting method defined in claim 5, includingthe step of maintaining an annular hot-zone in said casting chambersnear said inlet and extending toward said outlet, for the enhancement ofgravity segregation.
 15. The tube-casting method defined in claim 5wherein a lead-tin alloy is utilized as said liquid mold liningsubstance.
 16. A liquid mold lining for centrifugal casting of tube,consisting of molten lead-tin alloy.
 17. The method defined in claim 5,wherein said diameter control is effected by a combination of a partialvacuum inside said tube and a supra-atmospheric pressure outside saidtube.
 18. The method of claim 5, wherein said mold has not exit orificelip.
 19. A method as in claim 6, wherein the exit end of said tube issealed by capping.
 20. A method as in claim 10, wherein the exit end ofsaid tube is sealed by capping.
 21. A method for continuously castingtubing of a casting material of predetermined specific gravity andmelting temperature, comprising the following steps: rotating on agenerally horizontal axis, an elongated tubular mold having an inlet atone end and an outlet at its other end; injecting into the inlet of therotating mold, a liquid lining substance of higher specific gravity andlower melting point than said casting material and causing saidsubstance to acquire the form of a cylindrical lining within said moldin response to the rotation of the mold and resultant centrifugal force,and thereby providing a cylindrical casting chamber within the mold;injecting said molten casting material through said inlet into saidcylindrical casting chamber and causing it to assume the form of acylindrical tube in response to the rotation of said lining and causingsaid tube of molten casting material to be cooled by said lining and tothereby be congealed to a state within the range including solidifiedand semisolidified states, causing said lining to flow from the inlet tothe outlet exit orifice of the mold and to discharge from said orificewhile simultaneously causing said tube to exit freely through saidorifice while enveloped in the outflowing lining of liquid substance;causing controlled exit of said tube by mechanism external to saidorifice moving said tube at a predetermined speed; and controlling theoutside diameter of said tube substantially back of said orifice bymeans of back pressure in said liquid lining to a diameter smaller thansaid orifice when said tube is congealed to to state, said back pressurebeing determined by the rate of introduction of said liquid liningsubstance and the resistance to flow thereof in said mold